1. Field of the Invention
The present invention relates to a single electron device, and more particular, to a single electron device where electron islands are effectively formed using HSG (Hemi-spherical grained) silicon process, a method of manufacturing the same, and a method of simultaneously manufacturing a single electron device and an MOS transistor.
2. Description of the Prior Art
A single electron device is an ultimate structure of electronic device which can be controlled with one electron. Based on a transistor concept, single electron devices having a structure similar to an FET (Field Effect Transistor) have already been proposed and researched for implementing ultra high integrated memories or super low-power consumption circuits. In addition, various element structures and circuits with a new concept have been researched.
Hereinafter, operation principles of a single electron device will be explained with reference to FIGS. 1 and 2. FIG. 1 is a schematic view illustrating a single electron device having a structure similar to an FET.
An electron island 120 is surrounded by two tunnel junctions 115 and 125 and a capacitor 135, where the tunnel junctions 115 and 125 have resistance and capacitance characteristics of (R1, C1) and (R2, C2) respectively, and the capacitance of the capacitor 135 is Cg. A constant bias voltage V is applied to a node denoted by A and a control voltage, Vg is applied to a node denoted by B in FIG. 1. Such structure is very similar to that of an MOSFET, where two nodes A and C correspond to a source and a drain, respectively, and the input node B corresponds to a gate. The electron transport characteristic with such configurations is shown in FIG. 2 where current oscillation phenomena are manifested due to the Coulomb blockade effect.
That is, as shown in FIG. 2, an oscillating pattern of the current with respect to Vg having a period of e/Cg is obtained. FIG. 2 shows a relationship between the control voltage Vg for controlling the potential of the electron island 120 and the current, i flowing the electron island through tunnel junctions with the applied voltage V. Herein, a peak region represents a conducting state and a valley region represents an insulating state by the Coulomb blockade. Further, the period, e/Cg represents a transition between states of total charges differed by e, a single electron's charge, which means that a source-drain current can be modulated by induced charges smaller than e, a single electron's charge. Therefore, this device is called a single electron transistor.
When tunnel junctions 115 and 125 have resistance and capacitance values of (R1, C1) and (R2, C2) and the capacitor 135 has a capacitance of Cg, the conditions for a single electron behavior having the characteristic as shown in FIG. 2 are as follows;Ri>>h/e2˜26 kΩ (herein, I≈1, 2)  (Equation 1)e2/Ct>>kBT (herein, Ct=C1+C2+Cg)  (Equation 2)
The equation 1 describe the condition for the localization of a electron island 120 that can discriminate the tunneling electron one by one, and the equation 2 describes the condition that the electron tunneled into the electron island 120 can block other thermally activated electrons. As can be seen from the equation 1, the resistance of the tunnel junctions should be higher than hundreds of kΩ. As can also be seen from the equation 2, the size of the device should be smaller than tens of nanometers in order for the electron island 120 to have small capacitance enough for room temperature operation.
To meet those conditions, it is essential to develop the technology where electron islands with nanometers of size are uniformly formed using silicon in order to manufacture a single electron device compatible with conventional MOS transistor technologies. In current state of nanofabrication technique, it is only the electron beam direct lithography method to meet such a stringent requirement for sub-10 nm scale lithography, but it is not suitable for mass production.
Therefore it is urgent to develop new technique toward nanoscale fabrication processes especially for silicon in order to realize integrated circuits of single electron transistors which have merits in high integration and low power.